This invention relates generally to the creation of highlight color images and more particularly to the use of light lens imaging techniques for such purposes.
The invention can be utilized in the art of xerography. In the practice of conventional xerography, electrostatic latent images are formed on a charge retentive surface such as a photoreceptor by first uniformly charging the photoreceptor. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation. The areas of charge dissipated on the photoreceptor correspond to residual or background voltage levels. Thus, the photoreceptor contains two voltage levels in the case of a binary digital system. In the case of a light/lens system a whole array of voltage levels are present on the photoreceptor. This latent charge pattern is rendered visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
Many documents and printed materials consist of black, white (color of substrate) and one highlight color. It is a very important and formidable task to print and copy these documents.
U.S. Pat. No. 4,937,636 granted to Rees et al describes one technique that involves tri-level imaging in a light lens copier. In that patent, an original to be copied is modified using fluorescent ink on selected portions thereof. Light reflected from the modified original is transmitted through a colored filter and is projected onto the surface of a monopolar photoreceptor. Light incident on the fluorescent ink is absorbed over a specific wavelength range and is re-admitted at a higher wavelength. This light, and light reflected from the white background, are transmitted through a filter of a color associated with the re-emitted wavelength. Light reaching the photoreceptor discharges charged areas thereon at two energy levels. The resulting latent image incorporates three separate discharge levels corresponding to the black image information, color fluorescent areas, and background areas. The black and color areas are developed with appropriate colored toner by developer units biased at the appropriate levels. The requirement of modifying or creating an original using special inks presents a serious drawback to commercial acceptance of the process.
A commercially available printer (Xerox 4850) is capable of making highlight color prints in a single pass. That printer as in the case of the Rees device utilizes a technique know as tri-level imaging. Tri-level imaging splits the Photo-Induced-Discharge Curve (PIDC) thereby yielding low contrast images which allow only low development potentials. U.S. Pat. No. 4,078,929, R. Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color. In this scheme, the photoreceptor is initially charged to a voltage V.sub.0. It is then selectively discharged with a single Raster Output Scanner (ROS) to approximately V.sub.0 /2 in the background (white) areas and to near zero or residual potential in the color areas.
It is well known in the art to produce two color output copies using a multi-pass system, a present commercial example being the Canon 3625 copier. For this type of system, an operator utilizes a mimic-type electronic edit pad to delineate areas of an original it is desired to highlight. The coordinates of a selected area are entered into machine memory. During a first exposure cycle all areas on the photoreceptor are erased save the selected highlight color area which is then subsequently developed with the appropriate color toner. The resulting image is transferred to a copy sheet, fused and returned to the developer station entrance zone where it is re-registered. A second exposure of the original is made and the highlight area only is erased. The resulting latent image is developed with conventional black toner, and transferred to the copy sheet which is then fused and the copy sheet conveyed to an output tray.
The type of system exemplified by the Canon 3625 has several disadvantages; it requires an expensive electronic component, the edit pad as well as additional memory. Registration following the first exposure is difficult to achieve. Since the system is two cycle (two pass) the productivity is limited, It is therefore highly desirable for an electrophotographic reproduction machine to reproduce both the black and the color (red for most highlighting purposes) information in a single pass. By single pass, it is meant that a composite electrostatic latent image having regions corresponding to the red information and black information is recorded on the photoconductive surface. This composite electrostatic latent image is developed with black and red toner particles to produce a two-color toner powder image. This two-color powder image is subsequently transferred to the copy sheet and permanently affixed thereto, In this way, a highlighted color copy of the original document may be readily produced at relatively high speeds, automatically and in perfect registration.
Following is a discussion of other prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the patentability thereof, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 3,832,170 Nagamatsu et al., describes a photosensitive member having an insulating layer acting as a color filter. The photoconductive drum is divided into three segments, each segment corresponding to a different colored electrostatic latent image. These differently colored electrostatic latent images are then developed by toner particles complementary in color thereto. The toner powder images are then transferred to a copy sheet in superimposed registration to form a multicolor copy corresponding to the original document.
U.S. Pat. No. 4,078,929 Gundlach, discloses a reproduction machine which can form a two-color copy of an original document either using conventional light lens exposure techniques, or electronically. A charge pattern of a single polarity and having at least three different levels of potential is formed on a photoreceptor and developed in two colors by utilizing relatively negatively charged toner particles of one color and relatively positively charged toner particles of a second color. The exposure system requires the use of black and white images on an original document having a intermediate (grey) color.
U.S. Pat. No. 4,189,224 Sakai, discloses a photoconductive drum formed with first and second photoconductive layers of different spectral sensitivities. The photoconductive drum is charged and exposed causing electrostatic latent images to be formed on the respective layers according to the color within the original document. The charges of the latent images are of opposite polarity. Toner particles, similarly of opposite polarity, are used to develop the respective latent images. The toner particles of different colors. In this way, a two-color copy is formed.
U.S. Pat. No. 4,264,185 Ohta, describes an electrophotographic printing machine employing a photoconductive drum formed with at least two photoconductive layers of different spectral sensitivities. One layer may be panchromatic with the other layer being insensitive to red light. The drum is charged, at least twice, with opposite polarities to produce the charge pattern. A light image of the original document then exposes the charged regions of the drum. This results in positive and negative electrostatic latent images being recorded thereon. The latent images are developed with black and red toner particles of opposite polarity to form a two-color copy.
U.S. Pat. No. 4,335,194, Sakai, discloses a photoconductive member comprising a red sensitive photoconductive layer and a red-insensitive photoconductive layer. Two colors are printed by charging and exposing to white light, irradiating with red light and charging to an opposite polarity, charging to the same polarity as an opposite polarity, charging to the same polarity as the first polarity, and developing with red and black toners of opposite polarity.
U.S. Pat. No. 4,509,850, Weigl, teaches an electrophotographic printing machine capable of reproducing both black information and red information in a single pass. A continuously charged area and a modulated charged area are recorded onto a photoconductive surface. The modualted charged area is developed with polar or polarezable marking particles of a first color while the continuously charged area is developed with charged marking particles of a second color.
U.S. Pat. No. 4,479,242, Kurata, discloses a dichromatic reading device capable of separating a specific color from other colors of and original document. An electric circuit is provided which obtains the difference between the level of an image signal readout without a filter, and the level of a signal readout though a complementary filter for the specific color. Subsequently, an image signal for the specific color is produced from the difference.
U.S. Pat. No. 4,068,938, Robertson, teaches an electrophotographic printing machine capable of reproducing two-color copies from a two-color original document. A electrostatic latent image having three discrete potential levels is recorded onto a photoconductive drum. A high level is developed by particles of a first color corresponding to a dark color of the original document and a low level is developed by particles of a second color. The underdeveloped portion remains the color of the sheet of support material.
U.S. Pat. No. 4,562,129, Tanaka, discloses a bipolar photoreceptor which permits three different potential levels to be formed thereon.
From the discussion above it can be seen that those systems which utilize a single pass highlight color system (e.g. Gundlach) require the formation of separate and distinct levels of photoconductor charge. One level (high) corresponds to black information, intermediate level corresponds to white background and a third level (low) corresponds to the highlight color (red). The majority of the other prior art references cited above disclose some kind of bi-polar photoreceptor comprising multiple layers, each layer sensitive to a different color. The black and red discharge areas are then developed by developer units biased to appropriate levels. It would be desirable to enable a single pass color highlight system which does not require a bi-polar photoreceptor. It would also be desirable for the system to utilize a conventional light lens scanning system to make highlight color copies of originals having normal red and black images on a white background (unlike, for example, Gundlach which makes two color copies either from a CRT display or from an original with a grey background).